Separation of suspended particulates from fluids is a common filtration engineering task. Solid, semi-solid or gel-like particulates may be suspended in a gas or liquid in motion predominantly if the particulate density is close to the density of the surrounding fluid. Small water droplets and salt spray-mist, dust etc, may be suspended in the ambient air in motion. Rivers and large bodies of water may have floating particles, aquatic life, small fish egg and larvae in them. Induced local acceleration is used—typically by changing the local (small scale) flow pattern of the fluid—to induce inertial forces to the suspended particles. The inertial forces result in the partial or full separation of floating particles. The concentration of particles is low or close to zero in the filtrate stream and high in the concentrate stream. The efficiency of separation is commonly expressed as the ratio of particle concentration in filtrate stream over the particle concentration in the feed stream. There are several known inertial separation technologies. Demister vanes, marine vane separators, variety of cyclones, inertial spin tubes etc.
Wedge wire screens (WWS) are widely used in large water intake systems. The WWS is used to prevent debris, small fish and aquatic life from ingestion into the intake system. The symmetrical trapezoid shaped profile of the wire reduces the likelihood of screens from becoming plugged-up. The widening channels formed between the wires facilitate the free passage of water and suspended particles smaller than the gap (face-side opening) between the wires. The disadvantage of WWS is that it can only filter particles larger than the gap size between the wires and that it is prone to impingement of particles, organisms and objects on the outer surface of the screen. The other main disadvantage of the WWS is that suspended seaweed and long string-like algae can also entangle in it and cover up large surface areas causing flow restriction or blockage.
There are few existing technologies that employ sweeping flow to facilitate and improve the separation of suspended matter. The sweeping flow is a secondary component of the approach (or main inlet) flow. The sweeping component is tangential to the surface of the screen while the pass-through flow is perpendicular to the surface. The existing sweeping flow technologies utilize natural gravity or pump/circulator induced flow to create the required tangential flow over the surface of the screen. The sweeping flow technologies utilize wedge wire screens for inertial separation and for their reliable low-maintenance performance.
The disadvantage of the currently known wedge wire screens for sweeping flow applications is that their particle-separation efficiency is poor. The inertial forces determining the efficiency of separation are relatively low. The acceleration of flow—determining the separation forces—is reversely proportional to the curvature of streamlines within the boundary layer. The curvatures of streamlines entering the channels of the screen are relatively large. This results in a carry-through of considerable percentage of particles through the screen. To offset the lack of separation efficiency, small gap-size (more closed) screens are applied. The dense screen mesh however is susceptive to impingement of particles of larger size and screen surface clog-up.